Found this post on another group:
Under Tools|Options, there are two settings that affect this:
"When starting, go directly to my Inbox folder" and
"Send and receive messages at startup".
If *both* of those are enabled *and* the default mail account is
Hotmail, then OE opens the Hotmail Inbox at startup, which of course
automatically downloads any new headers, AND initiates a background
send/receive, which of course downloads any new messages in the Hotmail
Inbox, and thus duplicates are created. The problem appears to be that
those 2 functions don't communicate with each other.
Since both functions are actually downloading the same messages, and
since each message has its own unique message ID, manually deleting a
duplicate will result in *both* copies being deleted. If you already
have duplicates in a folder, I think the Synchronise commands should
remove the duplicate entries without deleting the message on the server.
To avoid the problem of duplicates, you should disable either of the 2
start-up options. If you want OE to open directly to the Inbox, then
disable "send and receive messages at start-up."
